1. Field of the Invention
The present invention relates to a power steering apparatus for reducing a steering force which is required for a driver to steer a steering wheel of a vehicle.
2. Discussion of the Prior Art
The assignee of the present invention has proposed a power steering apparatus of the type wherein a hydraulic pump is driven by an electric motor for generating an oil pressure. In the power steering apparatus of this type, a rotary valve of center-closed type is used as a directional control valve. The directional control valve is operated upon relative rotation between an input shaft connected with a steering wheel and an output shaft connected with a road wheel. While the steering wheel is maintained at its neutral position, the hydraulic pump is shut off from a power cylinder which generates a force for assisting the steering operation. When the steering wheel is steered so that relative rotation is effected between the input and output shafts, the hydraulic pump communicates with one of two cylinder chambers of the power cylinder to supply pressurized oil thereto. The pressure in a passage connecting the hydraulic pump and the directional control valve is compared to a higher one of the pressures in the two cylinder chambers of the power cylinder. When the detected pressure difference becomes larger than a predetermined value, the electric motor is stopped.
In such system, while the steering wheel is maintained at its neutral position so that the vehicle continues to travel straight, the electric motor is stopped. From the beginning of steering the steering wheel, oil pressure is immediately generated in one of the cylinder chambers of the power cylinder to assist the steering operation. Therefore, the energy consumed by the electric motor can be reduced and smooth steering operation can be realized.
FIG. 1 is a graph showing the relationship between drive voltage Vm output to the electric motor and the pressure difference .DELTA.P in the aforementioned power steering apparatus. While the pressure difference .DELTA.P is smaller than a reference value Pa, in other words, in the state that sufficient power assist cannot be obtained, a maximum drive voltage Va is applied to the electric motor. When the pressure difference .DELTA.P is between the reference value Pa and a predetermined value Pe, the drive voltage Vm is decreased in proportion to increases in the pressure difference .DELTA.P. When the pressure difference .DELTA.P is equal to or larger than the predetermined value Pe, the drive voltage Vm is lowered to zero to stop the electric motor.
In case where the pressure difference .DELTA.P does not sufficiently increase (in other words, it is fairly smaller than the predetermined value Pe) at the time when the steering wheel reaches the neutral position, the drive voltage Vm depending on that pressure difference .DELTA.P is applied to the electric motor. As a result, the pressure difference .DELTA.P increases, whereby the drive voltage Vm to the electric motor is decreased. In this manner, the pressure difference .DELTA.P reaches the predetermined value Pe, whereupon the electric motor is stopped. Namely, the drive voltage Vm is changed along the arrow 71 in FIG. 1. In the state that the steering wheel is maintained at the neutral position, the pressure difference .DELTA.P is maintained at the predetermined value Pe because the hydraulic pump is shut off from the power cylinder by the directional control valve of the normally center-closed type, so that the electric motor continues to be stopped.
However, the directional control valve of the aforementioned center-closed type cannot completely shut off the hydraulic pump from the power cylinder. Therefore, even when the steering wheel is maintained at the neutral position, with the electric motor being stopped, a small amount of the pressurized oil leaks to return to a reservoir. Such oil leak causes the pressure difference .DELTA.P to decrease, whereby the electric motor starts to be driven again. The electric motor is continuously rotated at such a super-low speed that the volume of the pressurized oil discharged from the hydraulic pump is equal to that of the oil leak. The rotation at the super-low speed of the electric motor makes the motor efficiency deteriorate notably. Consequently, the conventional system has a drawback that the electric motor continues to be used at a low efficiency, thereby resulting in the waste of battery power.